In agriculture, there is a need for finding the position of the boundaries of a field or other points of the field such as, for example, positions at which ground samples have been taken or are to be taken or problems to be eliminated occur such as stones to be removed or faulty drainage, as accurately as possible. These position data are used for planning and late control of the agricultural processing events. In other operations, too, such as an elimination of damages at known positions or for automatically directing or delivery of seeds and fertilizers to specific places, there is a need for accurately detecting a position. Further applications are also found in the building trade and in surveying.
In the prior art, it has been proposed to obtain the field boundaries by driving around the field with a vehicle whilst simultaneously detecting the position of the vehicle by means of a satellite-based positioning system (e.g. GPS, Global Positioning System) (see Published German patent application DE 43 42 171 A1). In this procedure, the vehicle must be moved precisely on the field boundaries. If this is not possible, e.g. because there are fences there, a distance is input and the driver attempts to drive always at the input distance from the field boundary. In this context, errors can never be avoided completely.
Furthermore, the possibility exists of equipping an employee with a portable device with which he goes over a field in order to record geographically referenced data from the field (P. Jurschik: Let a computer onto the field, Landtechnik March 1998, p. 142-143). The portable device is designed as a notebook or so-called pen computer and connected to an external DGPS receiver and where the correction signals are transmitted by radio waves from stationary transmitters. The operator goes over the field with the device in order to detect site contours there, take soil samples or create ratings.
At present, portable handsets which can be held in a hand, in the form of so-called smartphones or tablets are already equipped with DGPS receivers as standard. They are provided with differential correction signals from stationary transmitting devices (Published German patent application DE 100 54 764 A1). Similar devices are also used in surveying (Published German patent application DE 11 2010 000 674 A1).
The satellite-based positioning systems such as GPS, Glonass or the future Galileo use a number of satellites located in an Earth orbit and equipped with atomic clocks which in each case radiate electromagnetic (radio) waves containing time and identity or location information. If they are provided with a precise (atomic) clock, the associated receivers must in each case receive the signals from at least three satellites in order to be able to determine their current position in space. If the receiver does not have a precise clock, reception of signals from four satellites is required. The accuracy of the positioning increases with the number of satellites received in each case.
The accuracy of the positioning system is usually improved by the reception of radio signals containing correction data which are radiated by reference stations at known locations. These systems are called differential positioning systems (e.g. DGPS). The reference stations receive signals from satellites of the positioning system and derive from these position information which they convert into correction data and transmit by means of a radio transmitter to the vehicle which is equipped with a suitable receiver for the reception of the correction data. Using the correction data and the signals received from the satellites, the position of the vehicle is determined by a computer. On the one hand, so-called wide area systems having stationary reference stations with a relatively high transmitting power and, on the other hand, local reference stations are known which are mobile and, as a rule, are sited in the vicinity of the field to be processed in each case or are permanently mounted at elevated locations. An advantage of positioning by means of a local reference station compared with a wide area system lies in a much higher accuracy which is achieved because there are no network propagation times and the correction signal is generated in the immediate environment of the operating site.
A further possibility for improving the accuracy of the positioning consists in detecting signals of the satellites of the positioning system at different frequencies. Since atmospheric influences affect the receiving times due to their frequency-dependent dielectric constant, these influences can be compensated for by more elaborate multi-frequency receivers.